1. Field of the Invention
The present invention relates to a magnetooptic recording medium for recording information signals, and more specifically, to a magnetooptic recording medium for secondary information signals on a recording film in a vertical direction by irradiating an energy beam such as a laser beam under an external magnetic field.
2. Discussion of Related Art
Optical recording media having excellent properties such as large capacity, high reliability and long life have been developed. Among them, in particular, there is a great demand for a magnetooptic recording medium which is erasable by a user. In recent years, the expectation has been for the magnetooptic recording medium to have a larger capacity and a higher processing speed, since the quantity of information has steadily increased and become increasingly diverse. To attain a large capacity, methods for increasing a recording density are known, including a method for equalizing recording densities in all regions of a disk by using, for example, a ZCAV system, a method for increasing the linear density by using, for example, a mark edge recording system, and a method for increasing the number of tracks in the radial direction of a disk by narrowing the width of the tracks.
Pre-pits are formed on an optical disk substrate for use in a pre-format, such as address information, timing setting and tracking setting. When the tracks for recording have a narrow width, a signal to be reproduced from a track is more likely to be influenced by adjacent grooves and lands due to cross-talk or deficiency in transfer of pre-pits (hereinafter referred to as "pit deviation") during molding of a resin substrate. As a result, the reproduced signal may suffer variation. The variation in reproduced signals is a major error causing factor, resulting in various problems, such as increase in error rate. In the conventional art, reproduced signals have been processed, for example, by differentiation. Thus, the variation could be eliminated or decreased conveniently by means of the differentiation even when a small variation is given as in the conventional art. However, the variation remains as it is when using an original waveform slice process in which an original waveform itself is subjected to a slicing process to be converted into a binary system, which has been a problem.
Additionally, in the case of a format in which pre-pits are not aligned on adjacent tracks in a radial direction of a disk but exist at positions deviating in circumferential directions of the tracks, such as the ZCAV system, the pre-pits cause variation in reproduced signals in recording regions adjacent to or near the pre-pits, which has been a problem.
As the width of the tracks becomes narrower in accordance with the rapid advances in recent years to realize a high density, it becomes difficult to carry out signal processing, such as differentiation of reproduced signals. For this reason, it may be rather desirable to use the original waveform slice process for magnetooptic recording media having narrow tracks and high densities in recent years. However, even though this system encounters variation in reproduced signals, it cannot eliminate or decrease the variation, resulting in a problem in that correct conversion into a binary system is not performed. Thus, the variation is especially noticed as an important problem.
Additionally, in the conventional art, in the case of a format in which pre-pits are not aligned on adjacent tracks in a radial direction of a disk but exist at positions deviating in circumferential directions of the tracks, an influence of the pre-pits appears as variation in signals reproduced from recording positions where the pre-pits exist on adjacent or near tracks on a molded resin substrate. The variation appears in a signal detected at a zone boundary area in the case of the ZCAV system.
Some causes for occurrence of variation are a matter of speculations. Among them, it is especially pointed out as a major cause that turbulence occurs in a flow of a resin at positions where pre-pits are formed during injection molding of a resin substrate, resulting in the formation of a local portion having large birefringence in a recording area where the pre-pits exist on adjacent or near tracks. The birefringence generates retardation in polarized light which is reflected from the substrate. It has been hitherto considered that a positive correlation exists between transfer performance during molding of a resin substrate and C/N (Carrier To Noise Ratio). Therefore, molding conditions for a resin substrate have been designed to carry out satisfactory transfer. However, if the transfer rate is set to an extremely high value in consideration of the transfer performance, an excessive pressure is applied to the resin itself, which has exerted a number of bad influences, such as increase in turbulence of the resin flow due to formation of pre-pits and grooves, and occurrence of large birefringence due to solidification at a maintained high pressure.
The occurrence of retardation due to adjacent pre-pits is a source of trouble in the same manner as in the cross-talk and the pit deviation, because it varies reproduced signals of magnetooptic information. In order to avoid such variation in magnetooptic information signals which cause a signal error, it is important to exclude turbulence in the flow of resin at positions of pre-pits during injection molding of the resin substrate and form a uniform resin at recording positions where pre-pits exist on adjacent or near tracks.
Various methods are known for measuring the local turbulence of a resin. For example, the local turbulence of a resin, that is variation in retardation, varies reproduced signals of magnetooptic information. By using this fact, a magnetooptic recording film formed on the resin may be measured to determine a degree of the turbulence by using an apparatus for converting a rotational angle of polarization into a change in light amount. A value (A) is obtained by measuring a magnetooptic signal from a predetermined track as a change in light amount in the absence of a turbulence in the flow of the resin in the vicinity of pre-pits. A value (B) is obtained by measuring the same magnetooptic signal as a change in light amount in a region within 10 tracks from tracks including a cluster of pre-pits, that is a region subjected to variation in retardation due to the presence of the pre-pits. A value (a) is obtained according to an equation, (B)-(A)=(a).
It has been found that a resin substrate having a ratio (a/A) of not more than 20% can avoid problems due to the pre-pits. On the contrary, if a resin substrate has a ratio (a/A) of more than 20%, the turbulence of the resin becomes large, and local birefringence, i.e., retardation of reflected light, is apt to become large. Alternatively, in the case of measurement by using a variation amount of retardation measured with convergent light, a resin substrate is desirable in which the variation amount in an adjacent portion within 10 tracks from pre-pits is adjusted to be not more than 10 nm.